1. Field of the Invention
This invention relates generally to filters for use in subterranean environments, such as in oil and gas wells, water wells, and geothermal wells, and in other applications in which it is desired to remove a liquid or gas from the ground without bringing soil particulates, such as sand or clay, up with the liquid or gas. In particular, it relates to a filter having improved end connectors for sealing the ends of the filter.
2. Description of the Related Art
Filters are frequently used in subterranean wells in order to remove particulates from liquids or gases produced by the wells. Typical particulates which need to be filtered out are sand and clay, and for this reason, well filters for this purpose are often referred to as sand screens.
A well filter typically comprises a filter body mounted on the exterior of a pipe or other tubular support member. The filter is generally installed in series with a plurality of pipes forming a pipe string, and the filter is introduced into the well along with the pipe string. For example, in the case of an oil or gas well, the pipe string is a production string through which oil or gas is transported from a production zone within the well to the exterior of the well. When a pipe string containing the filter is being inserted into a well, the filter may come into contact with and catch on objects within the well bore. To prevent the filter body from sliding along the inner support member when such contact takes place, it is desirable to provide some restraint to lengthwise movement by the filter body. One method of restraint is to secure the filter body to annular metal end connector and to weld the end connectors to the inner support member. However, the inner support member of the filter is frequently a standard API grade of production pipe, which is normally made of a medium to high carbon grade of steel, whereas it may be desirable from the standpoint of corrosion resistance for the end connectors and filter body to be made of a different metal, such as austenitic stainless steel. It is notoriously difficult to weld steel with carbon contents of 0.20% and higher without causing hard spots, especially in the heat affected zone (HAZ), thus sacrificing corrosion resistance and sometimes impact properties of the base metal. Thus, there is a need for a well filter in which end connectors can be reliably secured to an inner support member, such as an API production pipe, without employing welding.
In some wells, such as geothermal wells, a well filter may be subjected to elevated temperatures during use and may undergo significant thermal expansion with respect to its dimensions when it is assembled. When the inner support member and the filter body of a well filter have different thermal expansion coefficients, or when the inner support member and the filter body are heated to different temperatures from each other, the difference in the amount of thermal expansion of the inner support member and the filter body may cause the filter body to be torn loose from the inner support member, allowing unfiltered fluids to bypass the filter body. Therefore, although it is desirable to prevent lengthwise movement of the filter body with respect to the inner support member during installation of the filter in a well, after the filter has been installed, it is desirable to enable relative lengthwise movement between the filter body and the inner support member when thermal expansion takes place.